1. Field of the Invention
The invention relates to a transmission mechanism, more particularly to a variable ratio transmission mechanism for an automobile steering system.
2. Description of the Related Art
A conventional variable ratio transmission mechanism, as disclosed in U.S. Pat. No. 7,278,512, is used in an automobile steering system. The automobile steering system includes a first steering shaft coupled to a steering wheel, and a second steering shaft coaxial to the first steering shaft and coupled to a pinion shaft that is connected to a wheel unit of the automobile. The conventional variable ratio transmission mechanism includes a first unit coupled to the first steering shaft, and a second unit that is driven by the first unit and that is coupled to the second steering shaft. The first unit includes a first housing connected co-rotatably to the first steering shaft, a rotor disposed in the first housing and coaxial to the first steering shaft, and a lock mechanism for locking the rotor. The second unit includes a connector connected to the rotor, and a second housing coupled to the connector and the second steering shaft.
When the lock mechanism is operated to lock the rotor from rotating relative to the first housing, the first steering shaft is rotatable with the first housing, the rotor, the connector, the second housing and the second steering shaft, so that the second steering shaft rotates in the same rotational velocity of the first steering shaft.
When the lock mechanism is operated to unlock the rotor from the first housing, the second steering shaft would be driven rotatably by the rotor and has a rotational velocity different from that of the first steering shaft. Consequently, under the same rotational velocity of the steering wheel, the rotational velocity of the second steering shaft can be adjusted to be suitable for high-speed or low-speed motion of the automobile, thereby ensuring safety of the steering action of the automobile.
However, since the rotor does not have a self-locking function, the lock mechanism is required to control the second steering shaft to rotate synchronously or non-synchronously relative to the first steering shaft, thereby resulting in a relatively complex structure and a relatively high manufacturing cost.